The present invention is directed to a method for self-calibration of frequency offsets in measurement equipment of an interference monitoring system. It is also directed to a self-calibrating device and an interference monitoring system.
Radio frequency interference (RFI) presents a serious threat for radio navigation systems and their augmentation systems like GPS, Galileo, EGNOS, WAAS, etc. Therefore, a continuous monitoring of the RFI environment is necessary to protect these systems and to generate situational awareness regarding possible system performance degradations caused by RFI.
FIG. 1 illustrates a conventional measurement chain applied in a sensing site of an interference monitoring system. This known measurement chain includes one or more antennas, a radio frequency (RF) front end and measurement equipment, such as signal analyzers and/or a combination of downconverters, analog/digital (A/D) converters and samplers. This equipment is typically remotely operated in order to avoid frequent site visits.
The measurement equipment makes use of an external or internal reference frequency source, e.g. a quartz oscillator. Such a quartz oscillator is susceptible to aging, so that the reference frequency provided by this device drifts away from its nominal value with time. This causes frequency offsets in the measurements provided by the measurement equipment. Therefore, this measurement equipment needs to be calibrated at regular intervals. It is therefore desired, to have such measurement equipment equipped with a self-calibration capability in order to avoid site visits for calibration purposes.
Calibration of measurement equipment such as signal analyzers is usually performed by the equipment manufacturer. In the context of interference monitoring this requires that either the equipment is shipped to the manufacturer for calibration, or that qualified personnel calibrate the equipment on site. Both approaches have drawbacks regarding interference monitoring system downtimes and costs.
Exemplary embodiments of the present invention provide a method for self-calibration of frequency offsets in measurement equipment of an interference monitoring system. Exemplary embodiments of the invention also provide a self-calibrating device employing such a method and an interference monitoring system with such a self-calibrating device. The self-calibrating device can employ the method by executing instructions stored in a non-transitory computer-readable medium.
An exemplary method of the present invention comprises the steps of:
a) sampling I/Q data by using the interference monitoring system measurement equipment;
b) acquiring navigation signals from the I/Q data;
c) estimating the satellite signal's carrier frequency;
d) calculating an expected satellite signal's carrier frequency;
e) comparing the expected satellite signal's carrier frequency with the estimated satellite signal's carrier frequency and calculating a frequency offset value as the difference between the two frequencies;
f) storing the frequency offset value in a memory; and
g) using the stored frequency offset value to compensate the frequency offset of at least one measurement in the future.
Thus, the invention involves a method that allows for a self-calibration of frequency offsets in the measurement equipment of an interference monitoring system.
The self-calibration method according to the invention involves sampling I/Q data using the interference monitoring system measurement equipment, to acquire signals from navigation satellites contained in this I/Q data, and to compare the estimated carrier frequencies of the navigation satellite signals with the corresponding expected carrier frequencies, which are calculated from satellite orbit data and the measurement equipment antenna position. The difference between estimated and expected carrier frequency equals the frequency offset of the measurement equipment at the expected carrier frequency, and allows compensation of the frequency offset for subsequent measurements.
In an advantageous modification of the inventive method the acquisition of navigation signals in step b) is carried out by correlating the sampled I/Q data with appropriate replica signals of different code phases and carrier frequencies.
In another advantageous embodiment of the inventive method the step of estimating the satellite signal's carrier frequency the satellite visibility is calculated from known ephemeris or almanac data.
The expected satellite signal's carrier frequency can be calculated from the position of the antenna of the measurement equipment, the satellite position and the satellite velocity.
In a further advantageous development of the inventive method the acquisition of the navigation signals from the I/Q data in step b) is carried out in two sub-steps:                b1) carrying out a coarse acquisition sub-step first and        b2) carrying out a refinement acquisition sub-step thereafter.        
An acquisition technique can be applied in the refinement acquisition sub-step with narrow frequency bins.
Alternatively, the I/Q data sampled in step a) are processed in the refinement acquisition sub-step in a tracking loop implementation initialized from the results of the coarse acquisition in sub-step b1).
It is advantageous to calculate the frequency offset value at more than one frequencies of interest.
The invention is also directed to a self-calibrating device which is adapted to carry out the steps described above.
Furthermore, the invention is directed to an interference monitoring system comprising measurement equipment provided with a self-calibrating device.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.